1. Field of the Invention
The present invention relates to a longitudinal magnetic recording medium with noise suppressed and stability improved, and more particularly to a magnetic recording apparatus having a high recording density which is arranged to use the longitudinal magnetic recording medium.
2. Description of the Related Art
In recent days, a request has been increasingly elevated for enlarging a volume of a magnetic disk drive. Accordingly, the magnetic head has been requested to have a far higher efficiency and the recording medium has been requested to have a far higher coercivity and lower noise.
The magnetic head is used of a composite head that includes both an inductive head for recording data and a spin-valve type head for reading back data. The spin-valve type head is a read-back head that is composed of a magnetoresistive sensor having a plurality of conductive magnetic layers whose directions of magnetization are relatively changed by the outside magnetic field so that a large resistance change may be brought about and conductive non-magnetic layers located between the adjacent conductive magnetic layers.
The magnetic recording medium is composed of a first underlayer called a seed layer formed on a substrate, a second underlayer composed of a Cr alloy having a body-centered cubic structure (bcc structure), a magnetic layer composed of a Co alloy having a hexagonal closed packed structure, and a carbon protective layer. In order to obtain a strong in-plane magnetic anisotropy (high in-plane coercivity), it is preferable that the longitudinal magnetic recording medium has a c-axis, that is, an axis of easy magnetization of the magnetic layer is oriented into the in-plane direction. Hence, the Co alloy of the magnetic layer has an orientation in which the (11.0) plane is positioned in parallel to the substrate plane (called the (11.0) orientation) or another orientation in which the (10.0) plane is positioned in parallel to the substrate plane (called the (10.0) orientation). It is known that the crystal lattice of the magnetic layer may be controlled by the seed layer. Further, it has been reported that the former orientation can be obtained by using Ta (see JP-A-4-188427) or MgO (see Appl. Phys, Lett., vol. 67, pp. 3638–3640, December (1993)) for the seed layer and the latter orientation can be obtained by using an NiAl alloy having a B2 crystal structure (see IEEE Trans. Magns., vol 30, pp. 3951 to 3953 (1994)) for the seed layer.
In order to further enhance the orientation of the magnetic layer, it has been studied that a non-magnetic Co alloy having a hcp structure is formed as a third underlayer between the second underlayer composed of a Cr alloy and the magnetic layer composed of a Co alloy. This study is tried as remarking the fact that the crystal of the magnetic layer is grown on the Co alloy underlayer having the same hcp structure as that of the magnetic layer more microfine than on the Cr alloy underlayer having a bcc structure. As this type of example, the CoCr alloy (see JP-A-10-79113 or JP-A-10-233014) or the CoCrRu alloy (see JP-A-2000-113445) has been reported.